This invention relates to a high efficiency, high frequency electronic oscillator-inverter circuit for starting and operating one or more electric discharge lamps. More particularly, the invention relates to a high frequency oscillator-inverter ballast circuit with an improved regenerative power supply.
U.S. Pat. No. 4,560,908 (12/24/85; Stupp et al) describes a high frequency oscillator-inverter circuit for starting and ballasting one or more discharge lamps and which includes a novel regenerative power supply. The Stupp et al apparatus exhibits a high efficiency and very high system power factor along with automatic regulation of the lamp current, reduced third harmonic distortion and substantial elimination of radio frequency interference (RFI). A limitation of this system is that at start-up, the circuit will sometimes go into an abnormal mode of operation during which both of the switching transistors simultaneously turn-off, whereupon the feed-choke which supplies energy to the oscillator-inverter will "fly-back" thereby generating a high voltage which overstresses the switching transistors such that they suffer severe damage, sometimes even causing complete destruction thereof.
One solution to this problem was to provide a transient absorption element, for example, a zener diode, which is coupled to the feed-choke to dissipate the fly-back energy generated therein during the start-up period. This approach is not entirely satisfactory since the zener diode or the like increases the cost of the apparatus. Furthermore, it is not entirely reliable because the fly-back voltage which will be generated, and must be dissipated, is not always easy to define in advance. Therefore, it requires considerable design and test time to determine the proper value of the zener diode, which further adds to the system cost.
In U.S. Pat. No. 4,560,908 (hereby incorporated by reference), the ballast system was provided with a regenerative power supply in order to supply power to the oscillator-inverter during the time period when the rectified unfiltered line voltage dropped below a given threshold value. In that system the regenerative power supply derived its energy from the main transformer of the high frequency oscillator-inverter. It has now been discovered that as a result of this method of operation, particularly at start-up of the high frequency oscillator-inverter, the primary winding of the main transformer was clamped to a voltage level which prevented the transfer of sufficient energy to the base drive winding of the switching transistors so as to maintain proper circuit oscillation. As mentioned above, the result of all of this will cause an abnormal mode of operation in which both switching transistors turn off and the feed-choke generates a high voltage which will damage the transistors unless the energy is dissipated by a transient absorption device.
It was further discovered that the start-up problem was due to the fact that the voltage of the resonant tank circuit of the oscillator-inverter was clamped to a low voltage by the feedback winding and by the input capacitor. The normal operating voltage of the capacitor is about 100 volts, but at start-up it is zero volts. A net DC current builds up in the feedchoke during start-up, and if the circuit is not able to sustain oscillation due to the lack of transferred drive current from the tank circuit to the transistors, both transistors turn off and a high flyback voltage is produced across the feedchoke in response to the abruptly diminished current therein. The flyback voltage must be clamped, and the energy in the feedchoke dissipated to protect the transistors and other circuit components.
Another possible solution to the above problem is to provide an auxiliary circuit which will pre-charge the power factor (PF) supply capacitor. This too is not an optimum solution because the auxiliary pre-charge circuit requires extra components which in turn increase the cost of the high frequency oscillator-inverter.